1. Technical Field
The present invention relates to a bipolar transistor with an embedded epitaxial external base region and a method of forming the same.
2. Description of Related Art
Millimeter wave and terahertz (THZ) applications will become the future development trend of wireless technologies, examples of which are millimeter wave communication, THZ communication, THZ imaging and the like. Currently, implementation of these applications mainly relies on Group III-V compound devices, and this has shortcomings such as a low integration level and a high cost. With the continuous development of the technologies, SiGe devices and technologies will become a competitor of the Group III-V compound devices. Currently, the SiGe technologies are widely used in such fields as communication, radar and high-speed circuits. The cut-off frequency Ft of the commercial SiGe process of the IBM has reached 350 GHz, and the maximum oscillation frequency Fmax of the SiGe devices developed by the European IHP has reached 500 GHz at the normal temperature. For the future millimeter wave and THZ applications, the SiGe devices still need to be continuously improved in performance, and this requires a novel structure for the SiGe devices.
External base regions of conventional bipolar transistors are usually processed through ion implantation and the performance of the resulting structures has shortcomings such as transient enhanced diffusion (TED) effect which will degrade the microwave performance of the devices. Some novel SiGe Heterojunction bipolar devices are formed by raised extrinsic base regions, but the external base regions beneath sidewalls in the resulting structures have a relatively large resistance, which reduces the microwave performance of the devices.